Re: [PATCH] arm64/smp: Move rcu_cpu_starting() earlier

[Paul E. McKenney]

On Thu, Oct 29, 2020 at 09:10:45AM +0000, Will Deacon wrote:
> On Wed, Oct 28, 2020 at 02:26:14PM -0400, Qian Cai wrote:
> > The call to rcu_cpu_starting() in secondary_start_kernel() is not early
> > enough in the CPU-hotplug onlining process, which results in lockdep
> > splats as follows:
> > 
> >  WARNING: suspicious RCU usage
> >  -----------------------------
> >  kernel/locking/lockdep.c:3497 RCU-list traversed in non-reader section!!
> > 
> >  other info that might help us debug this:
> > 
> >  RCU used illegally from offline CPU!
> >  rcu_scheduler_active = 1, debug_locks = 1
> >  no locks held by swapper/1/0.
> > 
> >  Call trace:
> >   dump_backtrace+0x0/0x3c8
> >   show_stack+0x14/0x60
> >   dump_stack+0x14c/0x1c4
> >   lockdep_rcu_suspicious+0x134/0x14c
> >   __lock_acquire+0x1c30/0x2600
> >   lock_acquire+0x274/0xc48
> >   _raw_spin_lock+0xc8/0x140
> >   vprintk_emit+0x90/0x3d0
> >   vprintk_default+0x34/0x40
> >   vprintk_func+0x378/0x590
> >   printk+0xa8/0xd4
> >   __cpuinfo_store_cpu+0x71c/0x868
> >   cpuinfo_store_cpu+0x2c/0xc8
> >   secondary_start_kernel+0x244/0x318
> > 
> > This is avoided by moving the call to rcu_cpu_starting up near the
> > beginning of the secondary_start_kernel() function.
> 
> Hmm, it's not really a move though -- we'll end up calling this thing twice
> afaict. It would be better to make sure we've called notify_cpu_starting()
> early enough. Can we do that instead?

It uses a per-CPU variable so that RCU pays attention only to the first
call to rcu_cpu_starting() if there is more than one of them.  This is
even intentional, due to there being a generic arch-independent call to
rcu_cpu_starting() in notify_cpu_starting().

So multiple calls to rcu_cpu_starting() are fine by design.

							Thanx, Paul